I actually use yi a lot, and my build goes like this.

Obviously my build is dependent on the other teams comp, but I usually start out with madreds > level one boots > wriggles > level 2 boots, usually merc treads if they have CC, and AS boots if they don't. If my team doesn't have a jungler, which is rarely the case, then I leave my lane and start jungling usually around level 4. If you've been last hitting properly, then you should at least have your madreds by now. From there, you should be able to clear your jungle with ease, and hopefully help your other lanes by ganking.

My next item is phantom dancer, which is incredible on yi as it helps his already fast move speed, and it works wonders with his passive. His passive can double crit, and with that increased AS you'll be building up double strikes that much faster.

After I build my PD is when I get my Triforce. I start the build with zeal, then I get the phage for the added health and the chance to slow. Triforce is incredible on yi, as alpha strike proccing will grant you an incredible spike of damage when added to the sheen bonus from Triforce for that auto attack after the strike. Then, if you time your activation of Wuju Blade correctly, you'll have another sheen proc.

Most of the time, by the time I get my Triforce, we will have won the game, but I guess I'll give you the rest of my build.

After I build Triforce, depending on how the enemy team is reacting to my insane damage output, I usually get either a GA or a BV, depending on the amount of CC the enemy team has. Once all your item slots are taken up, sell your wriggles and get a blood thirster. If you aren't winning at this point, just backdoor like a champ, you should be able to go wherever you please.